The single shot partial breast irradiation (SSPBI) trial was designed as a prospective Phase II "single-arm study". The use of a single dose tumor bed is expected to be very effective in terms of tumor control, but it could increase the incidence of radiation induced erythema. Therefore, the investigators assumed that a decreased DNA repair capability, as well as a reduced detoxification of the damage caused by oxidative stress could explain the increased acute toxicity, i.e. a higher incidence of erythema after a single dose. For this reason the investigators decided to investigate SNPs of genes involved in antioxidant and DNA damage repair pathways such as GST, XRCC1, XRCC3 and RAD51.